Method of purging and pumping vacuum chamber to ultra-high vacuum

ABSTRACT

A method for purging a vacuum chamber suitable for use in the production of integrated circuit structures on semiconductor wafers. The method comprises providing the chamber to be purged and flowing a heated, non-reactive gas, such as argon gas, through the chamber. The non-reactive gas is heated to a temperature of at least 90° C. Further, the chamber is heated to maintain it at a temperature of at least 90° C. while flowing the gas therethrough. Flowing the heated non-reactive gas through the chamber causes released impurities or contaminants to be efficiently swept from the chamber in the non-reactive gas flow. After flowing the heated gas through the heated chamber, the flow of gas is interrupted and the chamber, while still hot, is pumped down to a vacuum of about 5×10 -7  to determine whether or not the chamber has a leakage problem. The presence of a leakage problem may be determined by comparing the pumping to past pumping of similar sized chambers, or by measuring the partial pressure of common gases such as nitrogen and/or oxygen. If the partial pressure of oxygen is higher than about 5×10 -8  and the partial pressure of nitrogen is higher than 2×10 -7 , the vacuum chamber can be considered to have a leakage problem. Pumping times may, therefore, be shortened by the use of such screening for leakage problems while the vacuum chamber is still hot.

This is a continuation of copending application Ser. No. 08/084,938filed on Jun. 30, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the purging and pumping of a vacuum chamber,and more particularly to a method of pumping to ultra-high vacuum, avacuum chamber suitable for use in the manufacture of integrated circuitstructures.

2. Description of the Related Art

In the prior art, considerable effort has been devoted to providingultra-high vacuum capability in thin film processing equipment, forexample, for use in the formation of integrated circuit structures onsemiconductor wafers. Ultra-high vacuum is important for obtaining andsustaining low levels of contamination during production of theintegrated circuit structures. However, in pumping to ultra-high vacuum,for example, when PVD processing is practiced, it is important tominimize the period of time required because such time is non-productiontime. Further, when testing vacuum chamber equipment for ultra-highvacuum, it is desirable to predict the integrity of the vacuum chamberduring the pump to ultra-high vacuum levels to avoid wasted pumpingtime.

Various methods have been used for purging or decontaminating vacuumsystems, including electropolishing, baking at elevated temperatures,photon-stimulated desorption and glow discharge cleaning using oxygen ornoble gas. However, these approaches are not without problems. Forexample, baking is time consuming because a molecule released from asurface in the chamber is not efficiently removed to the chamber exit.Upon release, the desorbed molecule can take what is referred to as arandom path where it can strike other surfaces in the chamber with thechance of becoming re-adsorbed. On the other hand, glow dischargecleaning can result in further contamination by absorption of thecleaning gases into the chamber surfaces.

These gases are emitted later which leads to contamination. Also, it hasbeen reported that glow discharge cleaning may not be uniform,particularly in complicated chamber geometry.

Other systems have been reported which can reach ultra-high vacuum in amatter of minutes. Such systems employ expensive mirror finishes tominimize contaminants and water adsorption, glow-discharge cleaningusing super-dry nitrogen gas, and double turbomolecular pumps. However,fabrication of such systems is very costly and, accordingly, would beimpractical.

Thus, there is a great need for an economical method for purging avacuum chamber that would permit pumping to an ultra-high vacuum in arelatively short time, thereby ensuring a contaminant-free environment.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for removingcontaminants from a vacuum chamber utilizing a heated non-reactivepurging gas.

Yet, it is another object of the invention to provide a method forremoving contaminants using a combination of such heated non-reactivepurging gas and subatmospheric diffusion to entrain and remove desorbedcontaminants in the heated gas.

These and other objects will become apparent from a reading of thespecification and claims appended hereto.

In accordance with these objects there is provided an improved methodfor purging a vacuum chamber suitable for use in the production ofintegrated circuit structures on semiconductor wafers. The methodcomprises providing the chamber to be purged and flowing a heated,non-reactive gas, such as argon gas, through the chamber. Thenon-reactive gas should be heated to a temperature of at least about 90°C. Further, the chamber should be heated to maintain it at a temperatureof at least about 90° C. while flowing the gas therethrough. Flowing thenon-reactive gas through the chamber absorbs released impurities orcontaminants and efficiently sweeps them from the chamber in the gas.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a schematic outline of the equipment used to carryout the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises a method of purging and pumping of a vacuumchamber to remove adsorbed contaminants therefrom prior to pumping thechamber to ultra-high vacuum. Such a vacuum chamber is suitable for use,for example, in the manufacture of integrated circuit structures andother semiconductor devices on semiconductor wafers; or for use with anymaterial, fabrication, and/or processing requiring ultra high vacuum.

Referring now to the FIGURE a source 1 of a non-reactive gas such asargon is shown which flows through a heater 2 and a flow controller 3 toa chamber 4, such as a vacuum chamber, which is to be purged ofimpurities or contaminants. A vacuum pump 5, which may actuallycomprises several pumps, is also connected to chamber 4.

The supply of gas is heated by heater 2 prior to entering vacuum chamber4. Normally, the gas is heated to a temperature of at least 90° C.before entering vacuum chamber 4. For purposes of releasing adsorbedcontaminant molecules on inside surfaces of chamber 4, it is preferredthat the gas be heated to a temperature in the range of about 150° C. toabout 250° C. before entering chamber 4, with a typical temperaturebeing about 150° C. Heating the argon gas prior to entering chamber 4may be accomplished by the use of either inductive heaters or resistanceheaters comprising heater 2.

Flow controller 3 may comprise a gas flow meter placed in the stream ofgas flowing to chamber 4 to maintain a preset level of flow of heatedgas to chamber 4.

As shown in the FIGURE, the temperature of the gas flowing to the vacuumchamber may be monitored at 6, for example, by a thermocouple whichregulates the heat input to the gas so as to maintain it at a presetlevel.

Further, while the heated gas may be used as the source of heat beingapplied to the vacuum chamber, it is preferred that additional heat beadded to the vacuum chamber by an independent source comprising chamberheater 7. Thus, for purposes of providing or transferring additionalheat to vacuum chamber 4, independent heaters, such as lamps or chamberheaters or gas-line heaters, may be used. The independent heaters aid inmaintaining the gas temperature in the chamber typically in the range of90° C. to 250° C. while sweeping the non-reactive gas therethrough.

Heating the chamber, as well as heating the non-reactive gas, isimportant because of the energy required to break the bonding energythat holds or bonds the contaminating molecule to the surface of thechamber. It is this bonding energy which must be overcome before themolecule is released into the inside of the chamber. While such energymay be imparted to the contaminating molecule adsorbed on the chamberwall by virtue of the argon molecules hitting the chamber wall, this isnot efficient because of the length of time required to remove all ofthe adsorbed molecules. Similarly, heating the chamber with chamberheaters as noted earlier, for example, is not as efficient as usingheated gas in combination with the independent heaters. Thus, the firststep of the present process is to ensure the efficient release of thecontaminating molecule from the inside surfaces of the chamber. This isaccomplished by imparting to the chamber a level of energy sufficient toquickly and efficiently break the bonding energy of the contaminatingmolecules.

As shown in the FIGURE, for purposes of regulating the heat in thechamber, the chamber temperature may be measured at 9 with athermocouple so as to control the heat input from the independentheaters.

As noted earlier, the chamber and internal parts may be baked afterassembly of the vacuum system, but prior to introducing or flowing argongas therethrough. That is, the system is heated by the heaters, whileevacuating the chamber, but prior to any gas flow into the chamber. Suchbaking is normally designed to dislodge surface contaminants that areadsorbed to the material constituting the chamber. However, such bakingis not efficient in removing such dislodged contaminants from the vacuumchamber inside surfaces because of the lack of efficiency in removingsuch molecules, resulting in recombination of the dislodged contaminantwith the surface, i.e., readsorption. In fact, molecules desorbed bybaking frequently become adsorbed in another region of the chamber dueto the random path the desorbed molecules can take before exiting.Furthermore, the amount of energy needed for desorption of contaminantsoften cannot be supplied solely by such baking.

Therefore, in accordance with the invention, the second aspect of theprocess for efficiently purging the vacuum chamber of contaminants inaccordance with the invention involves preventing such contaminantmolecules from becoming re-adsorbed on the vacuum chamber surfaces afterthey have been released. This is accomplished by the heated gas streamflowing through the vacuum chamber where the heated gas contacts thereleased contaminant molecules. The heated gas flowing or sweepingthrough the chamber essentially prevents the contaminant molecules frombeing readsorbed on the inside surfaces of the chamber. Further, theflow of gas takes the contaminant molecules efficiently to the exit ofthe chamber with only minimal opportunity to re-adsorb.

The heated gas can be injected under pressure into the vacuum chamberand permitted to exit therefrom at atmospheric pressure. This maintainsthe chamber under a positive pressure. Or, the gas may be injected underpressure to the vacuum chamber and removed at sub-atmospheric pressure,together with desorbed contaminants, by the pumping system, in whichcase the chamber may still be purged under a positive pressure.Preferably, however, the heated gas is introduced into the vacuumchamber at relatively high, but subatmospheric, pressures and thenremoved at subatmospheric pressure, together with desorbed contaminants,by the pumping system. It is preferred to operate the process so as tomaintain the chamber under a subatmospheric pressure for severalreasons, including minimizing the escape of any toxic molecules that maybe present on the inside chamber surfaces. Thus, while the heated gas isbeing flowed through the chamber, the chamber is preferably maintainedat a pressure of from about 50 to about 750 Torr as measured at 8, asshown in the FIGURE.

This pressure may be maintained by a roughing vacuum pump system whichmay comprise one or several roughing pumps. The chamber pressure may bemeasured by a pressure gauge and relayed to the vacuum pump for purposesof maintaining the desired chamber pressure. In the preferred embodimentof the invention, the roughing vacuum pump is operated continuouslywhile gas is being flowed through the chamber. Maintaining asub-atmospheric pressure aids flow of gas through the chamber andconsequently aids in the removal of released contaminant molecules.

The flow of heated gas into the chamber is maintained at a level, forexample, equivalent to a minimum flow of at least about 5 standard cubiccentimeters per second (sccm) into a 5 liter chamber, and preferablyequivalent to a flow into a 5 liter chamber ranging from about 20 sccmto about 150 sccm to ensure a sufficient gas flow to accomplish thedesired sweeping of the desorbed contaminants out of the chamber.

While reference has been made herein to the use of heated argon as apurging gas, it will be understood that other gases, including otherinert gases, can be used. Such gases can include, for example, nitrogenor neon, as well as combinations of such gases. However, the use ofargon is preferred.

In operation of the process, the heated non-reactive gas is pumpedthrough the vacuum chamber for a predetermined period, following whichthe chamber is tested, while hot, to determine whether or not it iscapable of reaching and maintaining a high vacuum. This period may bedetermined empirically based on previously tested vacuum chambers (ofthe same size) capable of being pumped to ultra-high vacuum. Analysis ofthe partial pressures for oxygen or nitrogen, or any other common gases,may also be used in determining the pump down time required prior totesting. Then, after pumping or flowing the non-reactive gas through thevacuum chamber for the predetermined period of time, the flow of heatedgas to the chamber is stopped. The pressure or vacuum of the chamber ischecked, i.e., the chamber is checked for the presence of contaminants,by evacuating the chamber, while the chamber is still hot, to anultra-high vacuum, preferably in the range of from about 4×10⁻⁷ to about6×10⁻⁷ Torr, and typically about 5×10⁻⁷ Torr.

If the pressure is high compared to previously tested chambers at thispressure, this is indicative of a leakage problem with the chamber.Thus, further purging would not cleanse the chamber of oxygen ornitrogen, or other common gases, and further pumping would be futile.Alternatively, after the partial pressure check, if the readingscorrespond or are not unusually high compared to previously testedchambers, this indicates that the vacuum chamber does not leak andconforms to manufacturing specifications. The vacuum chamber may then bepumped to an ultrahigh vacuum. Thus, long periods of pumping time ondefective chambers are avoided. It will be seen that this method has theadvantage of markedly shortening the time for which chambers are pumpedby testing to find defective chambers before attempting to pump toultra-high vacuum.

Pumping to ultra high vacuum may be achieved by the first use of aroughing pump, as previously mentioned, followed by a pump down using acryogenic or turbo pump, or an equivalent high vacuum pumping system.The chamber may be pumped down to 5×10⁻⁹ Torr, for example, to achieveultra-high vacuum.

After the vacuum chamber has been pumped to an ultra-high vacuum, it maybe filled with a selected gas such as nitrogen for storage or shippingpurposes. Nitrogen is particularly useful for this purpose because itbonds more strongly with vacuum chamber surfaces than argon. The use ofnitrogen is beneficial because its presence prevents water moleculesfrom bonding with the chamber walls. Nitrogen has a bonding strength ofabout 40 milli-Joules/mole (mJ/mole), and argon has a bonding strengthof about 20 mJ/mole. Thus, water molecules which have a bonding strengthof about 100 mJ/mole do not have the same ability to displace nitrogenfrom the chamber walls as they would to displace argon.

The following example is still further illustrative of the invention.

EXAMPLE

For this example, a vacuum chamber, such as a chamber used in physicalvapor deposition, can be used such as, for example, a 5 liter vacuumchamber contained in an Applied Materials Endura™ multichambersemiconductor processing system available from Applied Materials, Inc.,Santa Clara, Calif. A source of argon gas can be attached to thechamber. Two vacuum pump stages would be attached to the chamber inorder to pump the chamber to ultra-high vacuum. The first vacuum pumpstage would be a roughing pump stage, which could comprise one or moreroughing pumps, such as a DC-25 BCS vane pump, available from Leybold,Inc, or a WSU-151 Roots Blower, also available from Leybold, Inc.; andthe second pumping stage would be a cryogenic pump, such as a CTICRYO-TORR8F available from CTI, Inc.

The argon gas can be heated to a temperature of about 150° C. beforebeing introduced to the chamber. The flow rate of the argon gas to thechamber would be at least about 5 sccm and sufficient, in combinationwith the pumping system, to maintain a pressure within the chamber offrom about 50 to about 750 Torr. The roughing pump can be operated toprovide a pressure in the chamber of 750 Torr while the argon gas isbeing flowed through it. Heaters, such as halogen lamps, can be appliedeither to the inside or the outside of the chamber to maintain thechamber at a temperature of about 97° C. After flowing the heated gasthrough the heated chamber the flow of gas can be stopped and thechamber pumped to 5×10⁻⁷ Torr. Then, the partial pressure of the gasleaving the vacuum chamber can be analyzed. If the partial pressure ofoxygen is higher than about 5×10⁻⁸ and the partial pressure of nitrogenis higher than 2×10⁻ 7, the vacuum chamber can be considered to have aleakage problem. Thus, attempting pumping to an ultra-high vacuum is notnecessary. If, however, the partial pressure of oxygen and the partialpressure of nitrogen are less than the above specified amounts, thechamber can be predicted to pump to an ultra-high vacuum of 5×10⁻⁹ Torrupon reaching ambient temperature.

From the above, it will be seem that the present invention provides animproved method for purging vacuum chambers which method can markedlyshorten the time spent in attempting to pump the chambers to anultra-high vacuum.

Having thus described the invention, what is claimed is:
 1. A processfor purging a vacuum chamber suitable for use in production ofintegrated circuit structures on semiconductor wafers comprising thesteps of:(a) providing a vacuum chamber to be purged, said chamberhaving an interior surface; (b) simultaneously;i) flowing through saidvacuum chamber from a first point in said chamber; and ii) pumping outof said chamber, through an exit spaced from said first point, anon-reactive gas heated to a temperature of at least 90° C.; and (c)maintaining said chamber at a temperature of at least 90° C. and at avacuum level in a range of about 50 Torr to about 750 Torr while flowingsaid heated non-reactive gas through said chamber, thereby sweepingimpurities from said chamber with said heated non-reactive gas as saidnon-reactive gas flows out of said chamber and substantially preventingreadsorption of molecules that have been desorbed from the interiorsurface of said chamber.
 2. The process of claim 1 wherein said heatednon-reactive gas is selected from the group consisting of argon,nitrogen, and neon.
 3. The process of claim 1 wherein said heatednon-reactive gas comprises argon gas.
 4. The process of claim 1 whereinsaid chamber is heated prior to flowing said heated gas therethrough. 5.The process of claim 1 wherein said gas is heated to a temperature in arange of from about 90° C. to about 250° C. prior to flowing said gasthrough said chamber.
 6. The process of claim 1 wherein said chamber ismaintained at a temperature in a range of from about 90° C. to about250° C.
 7. The process of claim 1 including the step of stopping saidgas flow to said chamber and applying vacuum of at least 4×10⁻⁷ Torr tosaid chamber to exhaust said non-reactive gas therefrom and to test saidchamber vacuum integrity.
 8. A process for purging a vacuum chambersuitable for use in production of integrated circuit structures onsemiconductor wafers comprising:(a) providing a vacuum chamber to bepurged, said vacuum chamber having an interior surface; (b) baking saidchamber in a temperature of 90° C. to 250° C. prior to flowing gasthereinto to desorb molecules adsorbed to the interior surface of thechamber; (c) flowing argon gas through said chamber from a first pointin said chamber, said argon gas heated in a temperature range of fromabout 90° C. to about 250° C.; and (d) maintaining said chamber at avacuum level in a range of 50 to 750 Torr and in a temperature range of90° C. to 250° C. while flowing said heated argon gas through saidchamber, thereby sweeping said desorbed molecules and other impuritiesfrom said chamber with said argon gas to an exit spaced from said firstpoint and substantially preventing readsorption of molecules that havebeen desorbed from the interior surface of said chamber.
 9. A processfor purging a vacuum chamber suitable for use in production ofintegrated circuit structures on semiconductor wafers comprising thesteps of:(a) heating a vacuum chamber to a temperature of at least 90°C., said chamber having an interior surface; (b) thereafter flowingthrough said heated vacuum chamber argon gas heated to a temperature ofat least 90° C.; and (c) removing said heated gas from said chamberwhile flowing said heated argon gas through said chamber and at a ratesufficient to maintain a pressure below 750 Torr, thereby sweepingimpurities from said chamber and substantially preventing readsorptionof molecules that have been desorbed from the interior surface of saidchamber.
 10. The process of claim 9 wherein said chamber is heated to atemperature in a range of from about 90° C. to about 250° C. prior toflowing said heated argon gas therethrough.
 11. The process of claim 9wherein said argon gas is heated to a temperature in a range of fromabout 90° C. to about 250° C. prior to being introduced to said vacuumchamber.
 12. The process of claim 9 wherein said chamber is maintainedat a vacuum level in a range of from about 50 to 750 Torr while saidheated argon gas is flowing therethrough.
 13. The process of claim 9wherein said chamber is maintained at a temperature in a range of 90° C.to 250° C. while said heated argon gas is flowing therethrough.
 14. Theprocess of claim 9 including the step of stopping said argon gas flow tosaid chamber and applying vacuum of at least 4×10⁻⁷ Torr to said chamberto exhaust said argon gas therefrom and to test said chamber vacuumintegrity.
 15. The process of claim 14 including the step of measuringpartial pressure of at least one of nitrogen and oxygen after reachingsaid vacuum of at least about 4×10⁻⁷ Torr in said chamber.
 16. Theprocess of claim 15 including the step of pumping said chamber to anultra-high vacuum of at least 5×10⁻⁹ Torr after said step of measuringsaid partial pressure of at least one of nitrogen and oxygen.
 17. Aprocess for purging a vacuum chamber suitable for use in production ofintegrated circuit structures on semiconductor wafers comprising thesteps of:(a) heating a vacuum chamber in a temperature range of fromabout 90° C. to about 250° C., said chamber having an interior surface;(b) thereafter flowing through said heated vacuum chamber argon gasheated in a temperature range of from about 90° C. to about 250° C.; (c)maintaining said chamber in a temperature range of from about 90° C. toabout 250° C. while flowing said heated argon gas through said chamber;and (d) removing said heated gas from said chamber while flowing saidheated argon gas through said chamber and at a rate sufficient tomaintain a pressure of from about 50 to about 750 Torr within saidchamber, thereby sweeping impurities from said chamber and substantiallypreventing readsorption of molecules that have been desorbed from theinterior surface of said chamber.
 18. The process of claim 17 includingthe step of stopping said heated argon gas flow to said chamber andapplying vacuum of at least about 4×10⁻⁷ Torr to said chamber to exhaustsaid argon gas therefrom and to test said chamber vacuum integrity. 19.The process of claim 18 including the step of measuring partial pressureof at least one of nitrogen and oxygen after reaching said vacuum of atleast about 4×10⁻⁷ Torr in said chamber.
 20. The process of claim 19including the step of pumping said chamber to an ultra-high vacuum of atleast 5×10⁻⁹ Torr after said step of measuring said partial pressure ofat least one of nitrogen and oxygen, if said measured partial pressureof oxygen does not exceed about 5×10⁻⁸ Torr and said measured partialpressure of nitrogen does not exceed about 2×10⁻⁷ Torr.
 21. A processfor purging a vacuum chamber suitable for use in the production ofintegrated circuit structures on semiconductor wafers comprising thesteps of:(a) heating a vacuum chamber in a temperature range of fromabout 90° C. to about 250° C., said chamber having an interior surface;(b) thereafter flowing through said heated vacuum chamber argon gasheated in a temperature range of from about 90° C. to about 250° C.; (c)maintaining said chamber in a temperature range of from about 90° C. toabout 250° C. while flowing said heated argon gas through said chamber;(d) removing said hated gas from said chamber simultaneous with saidflowing step and at a rate sufficient to maintain a pressure of fromabout 50 to about 750 Torr within said chamber, thereby sweepingimpurities from said chamber and substantially preventing readsorptionof molecules that have been desorbed from the interior surface of saidchamber; (e) stopping said gas flow to said chamber; and (f) applyingvacuum of at least 4×10⁻⁷ Torr to said chamber to exhaust saidnon-reactive gas and to test chamber vacuum integrity.
 22. The processof claim 21 including the step of measuring partial pressure of at leastone of nitrogen and oxygen after reaching said vacuum of at least 4×10⁻⁷Torr in said chamber.
 23. The process of claim 22 including the step ofpumping said chamber to an ultra-high vacuum of at least 5×10⁻⁹ Torrafter said step of measuring said partial pressure of at least one ofnitrogen and oxygen, if said measured partial pressure of oxygen doesnot exceed about 5×10⁻⁸ Torr and said measured partial pressure ofnitrogen does not exceed about 2×10⁻⁷ Torr.